This invention relates to integrated circuits having a Chip-Enable function. More particularly, this invention relates to an apparatus and a method for providing improved memory device performance by delayed internal power-down.
Prior-art in the area of delayed power-down operation includes circuitry for driving hard-disk drives of computers. One of the problems involving such disk-drive circuitry is deciding when to stop, or power-down, spinning of the disk platter. The trade-off is the power saved during intervals when the disk platter is stopped from Spinning versus the large surge in power consumption if the disk is stopped, then re-started.
A similar problem arises when designing automatic power-down circuitry for use in integrated circuits. Instead of power consumption by starting and stopping of the disk platter, power is consumed by transient voltages and currents within the integrated circuit during short disable cycles.
A standard control feature implemented in memory and other integrated-circuit devices is the Chip-Enable function. This control input is used to place the output drivers in a high impedance state and to disable DC current paths to reduce power dissipation. The read path is typically disabled (powered-down), which includes disabling blocks such as input buffers, sense amplifiers, and output stages. While in the disabled state, these circuit block outputs default to a fixed state (usually all "zeros" or all "ones"). When the device is re-enabled with the chip-enable signal, the states of the previously disabled blocks are brought from the default states to an activated condition. Returning the device to the activated condition causes transient current consumption (power consumption) even though there is no change with respect of the device state prior to the disable or power-down cycle.
The Chip-Enable function is very effective at reducing overall power consumption at low-frequency operation (long intervals between disable cycles). However, at high frequencies (short intervals between disable cycles) the transient current consumption increases the total power required by the device. The transient currents also lead to voltage drops on internal power buses and degraded performance due to those voltage drops.
There is need for a Chip-Enable function that prevents transient currents at high-frequency disable cycles and that also allows the disablement of DC current paths, thereby reducing power dissipation.